This adapter offers a way to seamlessly integrate next-safe-action with Better Auth. It provides a betterAuth() function that fetches the session, blocks unauthenticated requests, and injects fully-typed { user, session } data into the action context.

Requirements

• Next.js >= 15.1.0
• next-safe-action >= 8.4.0
• better-auth >= 1.5.0

Installation

npm i next-safe-action better-auth @next-safe-action/adapter-better-auth

Quick start

1. Set up Better Auth

Create your Better Auth server instance:

// src/lib/auth.ts
import { betterAuth } from "better-auth";

export const auth = betterAuth({
	// ...your config (database, plugins, etc.)
});

2. Create an authenticated action client

// src/lib/safe-action.ts
import { createSafeActionClient } from "next-safe-action";
import { betterAuth } from "@next-safe-action/adapter-better-auth";
import { auth } from "./auth";

export const actionClient = createSafeActionClient();

export const authClient = actionClient.use(betterAuth(auth));

3. Use it in your actions

// src/app/actions.ts
"use server";

import { z } from "zod";
import { authClient } from "@/lib/safe-action";

export const updateProfile = authClient
	.inputSchema(z.object({ name: z.string().min(1) }))
	.action(async ({ parsedInput, ctx }) => {
		// ctx.auth.user and ctx.auth.session are fully typed
		const userId = ctx.auth.user.id;

		await db.user.update({
			where: { id: userId },
			data: { name: parsedInput.name },
		});

		return { success: true };
	});

How it works

betterAuth() creates a pre-validation middleware for the safe action client's .use() chain:

1. Fetches the session by calling auth.api.getSession({ headers: await headers() })
2. Blocks unauthenticated requests by calling unauthorized() from next/navigation when no session exists
3. Injects typed context by passing { auth: { user, session } } to next(), merging it into the action context

unauthorized() and auth interrupts

The default behavior uses unauthorized() from next/navigation, which requires experimental.authInterrupts in your Next.js configuration:

// next.config.ts
import type { NextConfig } from "next";

const nextConfig: NextConfig = {
	experimental: {
		authInterrupts: true,
	},
};

export default nextConfig;

Custom authorization

Pass an authorize callback to customize the authorization flow. The session is pre-fetched and passed to the callback:

import { unauthorized } from "next/navigation";
import { betterAuth } from "@next-safe-action/adapter-better-auth";
import { auth } from "./auth";

// Role-based access
export const adminClient = actionClient.use(
	betterAuth(auth, {
		authorize: ({ authData, next }) => {
			if (!authData || authData.user.role !== "admin") {
				unauthorized();
			}
			return next({ ctx: { auth: authData } });
		},
	})
);

authorize callback parameters

• authData: the pre-fetched session data ({ user, session } | null)
• ctx: the current action context from preceding middleware
• next: call this to continue the middleware chain, pass { ctx } to inject context

Server Action cookies

If your actions call Better Auth functions that set cookies (e.g. signInEmail, signUpEmail), add the nextCookies() plugin to your Better Auth instance. Refer to the Better Auth documentation for more details.

// src/lib/auth.ts
import { betterAuth } from "better-auth";
import { nextCookies } from "better-auth/next-js";

export const auth = betterAuth({
	// ...your config
	plugins: [
		// ...other plugins
		nextCookies(), // must be the last plugin in the array
	],
});

API reference

betterAuth(auth, opts?)

Creates a middleware function for use with the safe action client's .use() method.

Parameters:

• auth: the Better Auth server instance (return value of betterAuth())
• opts?: optional object with an authorize callback for custom authorization logic

Returns: a middleware function compatible with .use()

Exported types

• BetterAuthContext<Options>: the context shape added by the middleware ({ auth: { user, session } })
• AuthorizeFn<Options, NextCtx>: the authorize callback signature
• BetterAuthOpts<Options, NextCtx>: the options object type for betterAuth

Documentation

For full documentation, visit next-safe-action.dev/docs/integrations/better-auth.

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License

MIT